1. Field of the Invention
The present invention relates to a toner for use in electrophotography. In addition, the present invention also relates to an oilless fixing method and a process cartridge using the toner.
2. Discussion of the Background
Electrophotography is typically applied to copiers, printers, facsimiles, and the like machines. In order to make such a machine smaller in size and easier in maintainability, a developing unit mainly including a developing device, a drum unit mainly including an electrostatic latent image member, and a process cartridge integrally combining the developing unit, the drum unit, and the like unit, have been proposed recently, especially for printers and facsimiles.
The process cartridge is preferably applied to a machine employing a one-component developing method, which has an advantage in downsizing of the machine. In the one-component developing method, a one-component developer (hereinafter referred to as a “toner”, unless otherwise described) is triboelectrically charged by a thickness control member (hereinafter referred to as a “blade”, unless otherwise described) or a developer bearing member (hereinafter referred to as a “developing roller”, unless otherwise described), and a thin layer of the toner is formed on the developing roller. The thin layer of the toner is conveyed to a developing area where the developing roller faces an electrostatic latent image bearing member so that an electrostatic latent image formed on the electrostatic latent image bearing member is developed with the toner to form a toner image.
A contact one-component developing method in which a developing roller directly or indirectly contacts an electrostatic latent image bearing member is widely used. The contact one-component developing method provides high quality images with good thin line reproducibility and image density uniformity.
In the contact one-component developing method, a toner is frictionally contacted with a toner bearing member, a charging member, or a photoreceptor. As a result, the toner and the surfaces of the toner bearing member and the photoreceptor may largely deteriorate with time especially under tough environmental conditions such as a high-temperature and high-humidity condition and a low-temperature and low-humidity condition. Therefore, there is a need for solving the above problems.
On the other hand, a fixing system (hereinafter referred to as an “oilless fixing system”) without an oil applicator, configured to apply an oil to a fixing member to effectively separate a recording medium therefrom, is widely used recently. Such an oilless fixing system produces images with smooth surface.
In order to satisfactorily separate a toner having sharply-melting property from a fixing member in the oilless fixing system, the toner may include a large amount of a wax. However, such a large amount of a wax may not be well dispersed in the toner, resulting in deterioration of mechanical strength of the toner. Therefore, a contradictory problem arises that a toner including a large amount of wax easily deteriorates by application of mechanical stresses in the one-component developing method.
To solve the above-described problems, the following techniques have proposed: increasing the molecular weight of a binder resin of a toner to enhance mechanical strength of the toner; reducing dispersion particle diameters of wax particles in a pulverization toner so as to be uniformly dispersed therein; etc. In addition, a technique of completely encapsulating a wax in a toner, by a wet granulation method such as a polymerization method, for example, and controlling particle diameter and dispersion state of the wax in the toner is also proposed.
However, a toner having both satisfactory resistance to mechanical stress in a one-component developing method and satisfactory fixing property in an oilless fixing system is not yet provided. For example, if the molecular weight of a binder resin is increased to enhance mechanical strength of a toner, the toner may not satisfactorily melt when fixed, resulting in production of full-color images with weak fixing strength and poor glossiness. Furthermore, if the dispersion particle diameters of wax particles are reduced so that the wax particles are uniformly dispersed in a pulverization toner, few amount of wax particles are exposed at the surface of the toner while wax particles present inside the toner hardly bleeds out, resulting in provision of insufficient fixing property. If the amount of a wax is increased for enhancing separability of a toner, the wax may not finely dispersed in the toner. As a result, a contradictory problem arises again that mechanical strength of the toner deteriorates.
A toner completely encapsulating a wax, manufactured by a wet granulation method, exerts its separability by bleeding out the wax therefrom. Therefore, such a toner is preferably used for a fixing system with low speed and high pressure. In other words, such a toner is difficult to be used for a high-speed machine.
As another approach, published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2003-207925 discloses a toner including a binder resin and a wax having an affinity to the binder resin. A relationship between the elution amount of the wax in the case of immersing the toner in hexane and that in the case of immersing the heated toner in hexane is defined. It is disclosed therein that the amount of the wax present at the surface of the toner and the dispersion state of the wax in the toner may be optimized by the defined relationship.
JP-A 2005-157343 also discloses a toner including a binder resin and a wax, wherein the elution amount of the wax in the case of immersing the toner in hexane is defined. It is disclosed therein that the toner satisfies both durability and oilless fixability. However, the wax may bleed out from the toner too slowly because the wax has too high an affinity to the binder resin so as to be finely dispersed therein. Consequently, the toner may not exert separability in a high-speed fixing system.
To improve the separability in a high-speed fixing system, the affinity of the wax to the binder resin may be decreased so that the wax is completely incompatible with the binder resin. In this case, the wax may not be finely dispersed in the binder resin in a typical kneading process for manufacturing a pulverization toner, and therefore a part of the wax may release from the toner and form a film thereof on image forming members, and mechanical strength of the toner may decrease. As a result, the toner may satisfy neither durability nor fixability in a one-component developing method.